Carbides

Inorganic Compound of Class 12

Carbides are binary compounds of carbon with elements of lower or about equal electronegativity. Carbides are classified into three types on the basis of chemical bonding. These are salt−like carbides, covalent carbide and interstitial or alloy like carbides. Carbides are generally prepared by heating the element or its oxide with carbon or hydrocarbon at very high temperatures.

Ba + 2C → BaC2

2Li + 2C → Li2C2

CaO + 3C → CaC2 + CO

4Li + C2H2 → Li2C2 + 2LiH

Salt−like carbides

These carbides are formed by the metals of groups IA, IIA and IIIA (except boron), coinage metals, zinc, cadmium and some lanthanides. Depending on the nature of the hydrocarbon formed during hydrolysis by water or dilute HCl, the ionic carbides can be further classified into the following types.

(i) Acetylides: These are the ionic carbides which yield acetylene on hydrolysis. These are regarded as derivatives of acetylene and are thus called acetylides. The alkali metals and copper, silver and gold form M2C2 type compounds. The alkaline earth metals, zinc and cadmium give MC2 type carbides. These carbides are believed to contain [C ≡ C]2− groups. Two types of crystal structures characterize these carbides. Both are tetragonal and are derived from sodium chloride structure by replacement of Cl− ions by  C22- ions.

(ii) Methanides: These carbides evolve methane on hydrolysis. These are regarded as derivatives of methane and are called methanides Al4C3, Be2C, Mn3C, etc. are some examples of methanides. These contain C4− groups.

(iii) Allylides: These carbides evolve allylene (methyl acetylene) on hydrolysis. The only example of this type is Mg2C3. It contains  C34-discrete groups.

(iv) Mixed carbides: These carbides yield a mixture of hydrocarbons on hydrolysis. Carbides of iron group, UC2 and ThC2 belong to this class.

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